Retaining ring having inner surfaces with facets

ABSTRACT

A retaining ring comprises a generally annular body. The body comprises a top surface, a bottom surface, an outer surface connected to the top surface at an outer top perimeter and the bottom surface at an outer bottom perimeter, and an inner surface connected to the top surface at an inner top perimeter and the bottom surface at an inner bottom perimeter. The inner surface comprises seven or more planar facets. Adjacent planar facets are connected at corners. The inner bottom perimeter comprises straight edges of the planar facets connected at the corners.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/259,089, file Apr. 22, 2014, the entire disclosure of which isincorporated by reference.

TECHNICAL FIELD

The present disclosure relates generally to chemical mechanicalpolishing of substrates, and more particularly to retaining rings foruse in chemical mechanical polishing.

BACKGROUND

An integrated circuit is typically formed on a substrate by thesequential deposition of conductive, semiconductive or insulative layerson a silicon substrate. One fabrication step involves depositing afiller layer over a non-planar surface, and planarizing the filler layeruntil the non-planar surface is exposed. For example, a conductivefiller layer can be deposited on a patterned insulative layer to fillthe trenches or holes in the insulative layer. The filler layer is thenpolished until the raised pattern of the insulative layer is exposed.After planarization, the portions of the conductive layer remainingbetween the raised pattern of the insulative layer form vias, plugs andlines that provide conductive paths between thin film circuits on thesubstrate. In addition, planarization may be needed to planarize adielectric layer at the substrate surface for photolithography.

Chemical mechanical polishing (CMP) is one accepted method ofplanarization. This planarization method typically requires that thesubstrate be mounted on a carrier or polishing head of a CMP apparatus.The exposed surface of the substrate is placed against a rotatingpolishing disk pad or belt pad. The polishing pad can be either a“standard” pad or a fixed-abrasive pad. A standard pad has a durableroughened surface, whereas a fixed-abrasive pad has abrasive particlesheld in a containment media. The carrier head provides a controllableload on the substrate to push it against the polishing pad. A polishingslurry, including at least one chemically-reactive agent, and abrasiveparticles if a standard pad is used, is supplied to the surface of thepolishing pad.

The substrate is typically retained below the carrier head by aretaining ring. However, because the retaining ring contacts thepolishing pad, the retaining ring tends to wear away, and isoccasionally replaced. Some retaining rings have an upper portion formedof metal and a lower portion formed of a wearable plastic, whereas someother retaining rings are a single plastic part.

SUMMARY

In one aspect, the disclosure features a retaining ring comprising agenerally annular body. The body comprises a top surface, a bottomsurface, an outer surface connected to the top surface at an outer topperimeter and the bottom surface at an outer bottom perimeter, and aninner surface connected to the top surface at an inner top perimeter andthe bottom surface at an inner bottom perimeter. The inner surfacecomprises seven or more planar facets. Adjacent planar facets areconnected at corners. The inner bottom perimeter comprises straightedges of the planar facets connected at the corners.

In another aspect, the disclosure features a carrier head comprising asubstrate receiving surface and a generally annular retaining ringsurrounding the substrate receiving surface. The retaining ringcomprises a top surface, a bottom surface, an outer surface connected tothe top surface at an outer top perimeter and the bottom surface at anouter bottom perimeter, and an inner surface connected to the topsurface at an inner top perimeter and the bottom surface at an innerbottom perimeter. The inner surface comprises seven or more planarfacets. Adjacent planar facets are connected at corners. The innerbottom perimeter comprises straight edges of the planar facets connectedat the corners.

In another aspect, the disclosure features a method of polishingcomprising creating a relative motion between a substrate and apolishing surface and restraining the substrate with a retaining ring.The retaining ring comprises a top surface, a bottom surface, an outersurface connected to the top surface at an outer top perimeter and thebottom surface at an outer bottom perimeter, and an inner surfaceconnected to the top surface at an inner top perimeter and the bottomsurface at an inner bottom perimeter. The inner surface comprises sevenor more planar facets. Adjacent planar facets are connected at corners.The inner bottom perimeter comprises straight edges of the planar facetsconnected at the corners. The relative motion between the substrate andthe polishing surface causes the retained substrate to contact two ormore facets of the inner surface of the retaining ring simultaneously.

Embodiments of the retaining rings, the carrier heads, and the methodsmay also include one or more of the following features. The bottomsurface comprises channels extending from the outer surface to the innersurface, and each channel comprises an end open to the inner surface ofthe body at a corner. The inner surface comprises a first number offacets and the bottom surface comprises a second number of channels. Thefirst number is a positive integer times the second number. The firstnumber equals the second number. The channels are oriented at an anglerelative to a radial segment extending through a center of the retainingring. Each corner extends from the top surface to the bottom surface.Each facet has a rectangular or square shape having a top straight edgeformed with the top surface, a bottom straight edge formed with thebottom surface, and two straight side edges along two corners. The innersurface comprises a total of 18 facets. The inner bottom perimeter has asymmetric shape. One or both of the inner top perimeter and the innerbottom perimeter forms a polygon. A distance between a center of theretaining ring and a planar facet is about 150 mm to about 155 mm. Thebody comprises a wearable material. A polishing liquid is supplied tothe polishing surface so that the polishing liquid flows through thechannels in the bottom surface and beneath the retaining ring to thesubstrate.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic perspective view of a retaining ring.

FIG. 2 is a schematic cross-sectional view of a carrier head.

FIG. 3 is a schematic planar bottom view of the retaining ring of FIG.1.

FIGS. 4 and 5 are cross-sectional views of retaining rings.

FIG. 6 is a schematic planar top view of a retaining ring with aretained substrate at different positions relative to the retainingring.

DETAILED DESCRIPTION

A retaining ring in a CMP apparatus has an inner surface that confinesmovement of a substrate being polished by the CMP apparatus. In aconventional retaining ring, the inner surface has a circular perimeter.

The retaining ring described herein has an inner surface formed ofmultiple planar facets, with adjacent facets joined at corners. In a topor bottom planar view of the retaining ring, the facets of the innersurface form a polygon or other shapes having straight edges. Theretaining ring can also include channels in a bottom surface that facesa polishing pad when the retaining ring is assembled in the CMPapparatus. The channels can facilitate transporting slurry between thesubstrate and the polishing pad. The locations of the facets and thechannels can be chosen such that each channel has an end open to theinner surface of the retaining ring at a corner of the retaining ring.In use, the retaining ring having these features, and other featuresdescribed below, can be in contact with multiple points of the outerdiameter of the substrate simultaneously. The retaining ring resistswear and has a long life expectancy. The polished substrate can havegood thickness uniformity.

Referring to FIG. 1, a retaining ring 100 is a generally an annular ringthat can be secured to a carrier head of a CMP apparatus. A suitable CMPapparatus is described in U.S. Pat. No. 5,738,574 and a suitable carrierheads are described in U.S. Pat. No. 6,251,215, and U.S. Pat. No.6,857,945, the entire disclosures of which are incorporated herein byreference. The retaining ring 100 fits into a loadcup for positioning,centering, and holding the substrate at a transfer station of the CMPapparatus.

As an example, FIG. 2 shows a simplified carrier head 200 onto which theretaining ring 100 of FIG. 1 is secured. The carrier head 200 includes ahousing 202, a flexible membrane 204, a pressurizable chamber 206, andthe retaining ring 100. The flexible membrane provides a mountingsurface 240 for a substrate 242. When the substrate 242 is mounted, themounting surface 240 can directly contact a back surface 244 of thesubstrate. In this example, the membrane 204 is clamped between theretaining ring 100 and the housing 202. In some implementations, one ormore other parts, e.g., clamp rings, can be used to hold the membrane204. The pressurizable chamber 206 is located between the membrane 204and the housing 202 can be pressurized, e.g., using a fluid (gas orliquid), to push a front surface 246 of the substrate 242 against apolishing surface 252 of a polishing pad 250 to polish the front surface246. In some implementations, the pressure in the chamber 206, and thusthe downward pressure of the flexible membrane 204 on the substrate 242,can be controlled using a pump (not shown) that is fluidly connected tothe chamber 206 though a passage 208 in the housing.

The retaining ring 100 is secured near the edge of the housing 202 toconfine the substrate 242 below the membrane 204. For example, theretaining ring 100 can be secured by mechanical fasteners 236, e.g.,screws or bolts, that extend through passages 238 in the housing 202into aligned threaded receiving recesses in an upper surface 112 of theretaining ring 100. Additionally, the top surface 112 can have one ormore alignment apertures positioned to mate to a corresponding pin onthe carrier head to allow proper alignment when the retaining ring 100is secured to the carrier head. When the retaining ring 100 is securedto the housing 202, the circumference of the top of the top surface 112can be substantially the same as the circumference of the housing 202 sothat no gap exists along an outer edge of the carrier head. Theretaining ring 100 is removable from the housing 202 and the rest of thecarrier head 200 as a unit, without requiring disassembling the housing202 or removing the housing 202 from the carrier head 200.

A drive shaft 220 can be provided to rotate and/or translate the carrierhead 200 across a polishing pad. In some implementations, the driveshaft 220 can be raised and lowered to control the pressure of a bottomsurface 114 of the retaining ring 100 on the polishing pad 250.Alternatively, the retaining ring 100 can be movable relative to thedrive shaft 220 and the carrier head 200 can include an internal chamberwhich can be pressurized to control a downward pressure on the retainingring 100, e.g., as described in U.S. Pat. Nos. 6,183,354 or 7,575,504,which are incorporated by reference.

Referring to FIGS. 1 and 3, the upper surface 112 of the retaining ring100 is flat and a lower surface 114 contains channels or grooves 130.Other than the channels 130, the lower surface 114 can be flat and canbe parallel to the upper flat surface 112. In this example, the lowersurface 114 includes twelve channels 130, although there can be adifferent number of channels, e.g., eighteen or more channels. When theretaining ring 100 is assembled in a carrier head, such as the carrierhead shown in FIG. 2, the lower surface 114 contacts the polishing pad.The channels 130 permit a polishing fluid, such as slurry, which caninclude abrasives or be abrasive-free, to flow underneath the retainingring to the substrate.

The upper surface 112 and the lower surface 114 can be separated by adistance, or the height of the retaining ring, of about 12.5 mm to about37.5 mm. The channels 130 can be generally straight, and extend from aninner surface 120 to an outer surface 110 of the retaining ring 100.Each channel 130 can have a width W (see, FIG. 4) of about 0.75 mm toabout 25 mm, e.g., about 3.125 mm.

The channels 130 on the bottom surface 114 can be distributed at equalangular intervals around the retaining ring 100. The channels 130 aretypically oriented at an angle a, e.g., between about 30° to about 60°,or about 45°, relative to a radial segment (R) extending through thecenter of the retaining ring 100.

Referring to FIG. 4, each channel 130 includes two vertical sidewalls132 and a rounded ceiling 134, so that the channel 130 lacks sharp innercorners. For example, the rounded ceiling 134 can have a semicircularcross-section with a diameter equal to the distance between the twosidewalls 132. The two vertical sidewalls 132 can have the same height,e.g., of about 0.5 mm to about 3.75 mm. The vertical sidewalls 132 canhave a rounded or sharp junction with the ceiling 134. As anotherexample, shown in FIG. 5, the ceiling of the groove could have agenerally horizontal flat section 136, and the curvature 138 could belocated just the corners where the ceiling intersects the sidewalls.Referring again to FIGS. 1 and 3, at least the portion of the outersurface 110 of the retaining ring 100 adjacent the lower surface 114 canbe a vertical cylindrical surface having a circular shape in a top orbottom planar view. In some implementations, the retaining ring 100includes an overhanging portion 146 that has a larger outer diameterthan a bottommost portion 144, so that the outer surface 110 includes aledge 140.

Instead of a cylindrical surface, the inner surface 120 is formed ofmultiple facets 150, including facets 150 a, 150 b, 150 c, 150 d, 150 e,150 f (not all shown), each being a flat vertical surface and joining anadjacent facet at a corner, e.g., the corners 162, including 162 a, 162b, 162 c, 162 d, 162 e (not all shown or labeled). Thus, each corner 162can be a straight vertical corner.

The facets intersect the upper and lower surfaces 112, 114 alongstraight upper and lower edges 170 and 194, respectively. The straightedges of the facets connect to one another at the corners. Thus, in atop or bottom planar view, the connected edges can form a polygon.

In the example of FIG. 1, the inner surface 120 has twelve facets 150that form twelve lower, straight edges 170, including edges 170 a, 170b, 170 c, 170 d, 170 e, 170 f, 170 g, 170 h, 170 i, 170 j, 170 k, 170 l,with the lower surface 114 and twelve upper, straight edges 194,including edges 194 a, 194 b, 194 c, 194 d, 194 e, 194 f (not allshown). However, there the retaining ring 100 could have 12 to 148facets.

On average, the inner surface 120 and the outer surface 110 areseparated by a distance, or an average width of the retaining ring 100,of about 2.5 cm to about 5.0 cm.

Each channel 130 in the upper surface 114 has an end open to the innersurface 120 and an opposite end open to the outer surface 110. Theopenings in the inner surface 120 are located at the corners betweenadjacent facets. In some implementations, all openings of the channels130 in the inner surface 120 are located at the corners and no openingsof the channels 130 in the inner surface 120 are between two cornersalong an edge of a facet. In the example shown in the figures, eachcorner corresponds to one opening of a channel, and therefore, a channel130. The total number of facets in the inner surface 120 can equal thetotal number of channels in the lower surface 114. In someimplementations, the number of facets in the inner surface equals aninteger, e.g., 1, 2, 3, . . . , times the total number of channels. Forexample, a channel opening, and therefore, a channel 130, may be formedevery two, three, or more corners on the inner surface 120.

The inner surface 120 of the retaining ring 100, in conjunction with thelower surface 240 of the flexible membrane 204, defines a substratereceiving recess 300. The retaining ring 100 prevents the substrate 242from escaping the substrate receiving recess 300. Generally, thesubstrate is circular and has a diameter of about 200 mm to about 300mm. The size of the recess 300 in a top or bottom planar view isgenerally larger than surface area of the substrate 242 such that thesubstrate 242 can move its position relative to the retaining ring 100.For the purpose of discussion, an inner radius (IR) of the retainingring 100 is defined in the planar view of the retaining ring to be adistance between the center C of the retaining ring 100 to a centerpoint on an edge of a facet between two adjacent corners. The innerradius is larger than half of the substrate diameter, or the substrateradius, and can be e.g., of about 150 mm to about 155 mm.

Referring to FIGS. 2 and 6, during a polishing process, the carrier head200, including the housing 202, the membrane 204, and the retaining ring100, moves relative to the polishing pad 250. The substrate 242 followsthe movement of the carrier head 200 within the recess 300 defined bythe inner surface 120 of the retaining ring 100 and the membrane 204. Ata certain time moment during the movement, the substrate 242 contacts atleast two facets of the inner surface 120. For example, as shown in FIG.6, at a time moment when the substrate 242 is in a contacting position“1” relative to the retaining ring, the outer surface of the substratesimultaneously makes one contact with the facet 170 k and anothercontact with the facet 170 l (the contact locations are indicated by asymbol “x” and labeled with the position “1”).

Assuming that at the two contact locations, the substrate 242 does notslip with respect to the inner surface 120 of the retaining ring 100, atthe two contact locations, the substrate 242 and the inner surface 120have the same linear velocity. However, because the radius of thesubstrate 242 is smaller than the radius of the inner surface 120 of theretaining ring 100, at the two contact locations, the substrate 242 andthe inner surface 120 have different angular velocities. As a result,after this moment in which the substrate 242 is at the contactingposition “1”, the position of the substrate 242 relative to theretaining ring 100 changes and the contact locations between thesubstrate 242 and the inner surface 120 change. In other words, thecenter of the substrate 242 moves, e.g., rotates, relative to the centerof the retaining ring. For example, at another moment, the substrate 242is in a contacting position “2” relative to the retaining ring. In thisposition “2”, the outer surface of the substrate 242 simultaneouslymakes one contact with the facet 170 i and another contact with thefacet 170 j. At yet another moment, the substrate 242 is in a contactingposition “3” relative to the retaining ring. In this position “3”, theouter surface of the substrate makes one contact with the facet 170 hand another contact with the facet 170 g.

Although FIG. 6 shows that the substrate 242 is simultaneously incontact with two facets at each contacting position, e.g., the position“1”, “2”, or “3”, it is possible that the substrate 242 simultaneouslycontacts more than two facets, e.g., three or more facets, at eachcontacting position. For example, the number of contacting facets ateach contacting position can be controlled by controlling the totalnumber and/or shape of facets in the inner surface 120 of the retainingring, the radius difference between the substrate 242 and the innersurface 120, etc.

In some examples, each facet can have the same dimensions and shape, andthe planar view of the inner surface 120 can be highly symmetric. Inother examples, the facets can have different dimensions or shapes, andthe cross-section of the inner surface can be non-symmetric. Forexample, although the figures show that the facets are generallyrectangular or square, the facets can also have other shapes, e.g.,trapezoid. The corners shown in the figures extend from the uppersurface 112 to the bottom surface 114. In other examples, some cornersbetween two adjacent facets do not extend fully between the two surfaces112, 114.

By allowing the substrate 242 to be simultaneously in contact withmultiple facets at each contacting position, the wear of the innersurface 120 of the retaining ring can be reduced as compared to forminga single contact at each contacting position. For example, when an innersurface of a retaining ring is cylindrical, e.g., like the outer surface110 of the retaining ring 100, at each contacting position, a substratehaving a circular outer perimeter contacts the inner surface at a singlecontacting location. The multiple contacts can allow force between thesubstrate 242 and the inner surface 120 to be absorbed at the multiplecontacting locations and reduce the wear caused by the force at eachindividual contacting location. Furthermore, the contact angle in thesingle contact between a substrate and a cylindrical surface is normalto the inner diameter of the cylindrical surface. However, when thesubstrate 242 makes multiple contacts with the inner surface 120 atmultiple facets, the contact angles are not necessarily normal to theinner radius of the inner surface 120. As a result, the wear on theretaining ring at each contact location is reduced as compared to acontact having a contact angle of 90 degrees. The reduced wear can allowthe retaining ring to have an increased life expectancy.

During the relative movement between the retaining ring 100 and thesubstrate 242, the substrate makes no direct point-to-point contact withany of the channels 130 or channel openings located at the corners ofthe inner surface 120. Instead, the substrate only makespoint-to-surface contacts with the facets of the inner surface 120.Generally, the channels 130 form high stress areas in the retaining ring100, where the retaining ring tends to be damaged or break more easilythan other parts of the ring. By eliminating direct point-to-pointcontacts between the channels 130 and the substrate 242, the high stressareas are protected from direct impact of the force between thesubstrate 242 and the inner surface 120. The possible damages to theretaining ring from such force are reduced. As a result, the wear of theretaining ring is reduced and the retaining ring can be used for a longperiod of time.

In some polishing processes, the relative movement between the substrate242 and the retaining ring 100 can reduce asymmetry in a polishedsubstrate and improve within-wafer uniformity. In a polished substratewith asymmetry, the polished substrate has a thickness variation thatvaries with the angular coordinate. Without being limited to anyparticular theory, as compared to a single contact situation, themultiple contacts between the substrate 242 and the retaining ring 100can allow the substrate 242 to rotate to the carrier head, thusangularly spreading the effect of any asymmetric pressure distributionfrom the carrier head, and thereby reducing the chance of asymmetryoccurrence or the amount of asymmetry.

The retaining ring 100 can be formed from a material that is chemicallyinert to the CMP process. The material should be sufficiently elasticthat contact of the substrate edge against the retaining ring 100 doesnot cause the substrate to chip or crack. However, the retaining ring100 should not be so elastic as to extrude into the substrate receivingrecess 300 when the carrier head puts a downward pressure on theretaining ring 100. The retaining ring 100 should also be durable andhave a low wear rate, although it is acceptable for the retaining ring100 to wear away.

For example, the retaining ring 100 can be made of a plastic that ischemically inert in a CMP process. The plastic can have a durometermeasurement of about 80-95 on the Shore D scale. In general, the elasticmodulus of the plastic can be in the range of about 0.3-1.0×10⁶ psi.Suitable plastic can include (e.g., consist of) polyphenylene sulfide(PPS), polyaryletherketone (PAEK), polyetheretherketone (PEEK),polyethylene terephthalate (PET), polybutylene terephthalate (PBT),polytetrafluoroethylene (PTFE), polybenzimidazole (PBI), polyetherimide(PEI), polyetherketoneketone (PEKK), or a composite material. Anadvantage of polyphenol sulfide (PPS) is that it is reliable andcommonly used material for retaining rings.

The retaining ring 100 can also have other features or featuresalternative to those discussed above. In some implementations, theretaining ring 100 has one or more through holes that extendhorizontally or at a small angle from horizontal through the body of theretaining ring from the inner surface to the outer surface for allowingfluid, e.g., gas or liquid, to pass from the interior to the exterior,or from the exterior to the interior, of the retaining ring duringpolishing. The through-holes can be evenly spaced around the retainingring.

Although the side walls of the retaining ring 100 are illustrated asbeing vertical, the retaining ring 110 can include other features, suchas a lip or recess on the outer surface to assist with centering theretaining ring in a substrate loader or to provide a hard stop for theretaining ring against the top inner edge of a surrounding ring. In someimplementations, the inner surface and/or outer surface of the retainingring 100 can be tapered.

Although the retaining ring described above contains a single ring, insome implementations, a retaining ring can have a combination of two ormore ring portions stacked on top of each other. In suchimplementations, the retaining ring 100 described above can be thelowest ring portion having its lower surface in contact with a polishingpad. Certain parts of the retaining ring 100 may be altered to connectto the other ring portions.

Other features related to retaining rings and retaining ring portionsare described in U.S. Patent Application Publication No. 2005/0126708and U.S. Patent Application Publication No. 2013/0035022. The entirecontents of both applications are incorporated herein by reference.

Other features are in the claims.

1. A retaining ring comprising: a generally annular body comprising atop surface; a bottom surface; an outer surface connected to the topsurface at an outer top perimeter and the bottom surface at an outerbottom perimeter; and an inner surface connected to the top surface atan inner top perimeter and the bottom surface at an inner bottomperimeter, the inner surface comprising seven or more planar facets,adjacent planar facets being connected at corners, and the inner bottomperimeter comprising straight edges of the planar facets, the straightedges of the adjacent planar facets connected at the corners such thatthe inner bottom perimeter defines a polygon.
 2. The retaining ring ofclaim 1, wherein the bottom surface comprises channels extending fromthe outer surface to the inner surface, and wherein each channelcomprises an end open to the inner surface of the body at a samelocation relative to a nearest corner.
 3. The retaining ring of claim 2,wherein the inner surface comprises a first number of facets and thebottom surface comprises a second number of channels, and wherein thefirst number is a positive integer times the second number.
 4. Theretaining ring of claim 2, wherein the first number equals the secondnumber.
 5. The retaining ring of claim 2, wherein the channels areoriented at an angle relative to a radial segment extending through acenter of the retaining ring.
 6. The retaining ring of claim 1, whereineach corner extends from the top surface to the bottom surface, andwherein each facet has a rectangular or square shape having a topstraight edge formed with the top surface, a bottom straight edge formedwith the bottom surface, and two straight side edges along two corners.7. The retaining ring of claim 1, wherein the inner surface has betweentwelve and one-hundred forty-eight facets.
 8. The retaining ring ofclaim 1, wherein the inner bottom perimeter has a symmetric shape. 9.The retaining ring of claim 1, wherein the inner top perimeter forms apolygon.
 10. The retaining ring of claim 1, wherein a distance between acenter of the retaining ring and a planar facet is about 150 mm to about155 mm.
 11. The retaining ring of claim 1, wherein the body comprises awearable material.
 12. A carrier head comprising: a substrate receivingsurface; a generally annular retaining ring surrounding the substratereceiving surface, the retaining ring comprising a top surface; a bottomsurface; an outer surface connected to the top surface at an outer topperimeter and the bottom surface at an outer bottom perimeter; and aninner surface connected to the top surface at an inner top perimeter andthe bottom surface at an inner bottom perimeter, the inner surfacecomprising seven or more planar facets, adjacent planar facets beingconnected at corners, and the inner bottom perimeter comprising straightedges of the planar facets, the straight edges of the adjacent planarfacets connected at the corners such that the inner bottom perimeterdefines a polygon.
 13. The carrier head of claim 12, wherein the bottomsurface comprises channels extending from the outer surface to the innersurface, and wherein each channel comprises an end open to the innersurface of the body at a corner.
 14. The carrier head of claim 13,wherein the inner surface comprises a first number of facets and thebottom surface comprises a second number of channels, and wherein thefirst number is a positive integer times the second number.
 15. Thecarrier head of claim 14, wherein the first number equals the secondnumber. 16-18. (canceled)
 19. The retaining ring of claim 1, wherein thefacets are at equal angular intervals around the inner surface.
 20. Theretaining ring of claim 1, wherein the facets have the same shape. 21.The retaining ring of claim 1, wherein the facets are perpendicular tothe bottom surface.
 22. The retaining ring of claim 1, wherein thestraight edges have the same length.
 23. The retaining ring of claim 1,wherein the facets are rectangular.